The beautiful specimen of Phacops caudatus, which we figure in Plate V., has long been in the private collection of Professor Tennant. It is from the Upper Silurian limestone of Dudley, and would be a perfect example but for the exception only of its having the segments of the body somewhat bent inwards, and slightly distorted from their natural positions by pressure. The lateral edges and their segments are unfortunately obscured by being embedded in the matrix, so that they cannot be accurately delineated.

This characteristic species of Trilobite was first noticed by Brunnich, in 1781, under the name of Trilobus caudatus. It was subsequently termed Asaphus caudatus by Brongniart, Dalman, Dr. Buckland, and other writers. Burmeister, however, in his valuable work on the “Organization of Trilobites,” called it Phacops caudatus, which generic determination modern palæontologists have followed.